The present invention relates to a sensor apparatus for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable.
The process and storage industries have long used various types of equipment to measure process parameters such as level, flow, temperature, etc. A number of different techniques (such as mechanical, capacitance, ultrasonic, hydrostatic, etc.) provide measurement solutions for many applications. However, many other applications remain for which no available technology can provide a solution, or which cannot provide such a solution at a reasonable cost. For many applications that could benefit from a level measurement system, currently available level measurement systems are too expensive.
In certain applications, such as high volume petroleum storage, the value of the measured materials is high enough to justify high cost level measurement systems which are required for the extreme accuracy needed. Such expensive measurement systems can include a servo tank gauging system or a frequency modulated continuous wave radar system.
There are many applications that exist where the need to measure level of the product is high in order to maintain product quality, conserve resources, improve safety, etc. However, lower cost measurement systems are needed in order to allow a plant to instrument its measurements fully.
Further, there are certain process measurement applications that demand other than conventional measurement approaches. For example, applications demanding high temperature and high pressure capabilities during level measurements must typically rely on capacitance measurement. However, conventional capacitance measurement systems are vulnerable to errors induced by changing material characteristics. Further, the inherent nature of capacitance measurement techniques prevents the use of such capacitance level measurement techniques in vessels containing more than one fluid layer.
Ultrasonic time-of-flight technology has reduced concerns regarding level indications changing as material characteristics change. However, ultrasonic level measurement sensors cannot work under high temperatures, high pressures, or in vacuums. In addition, such ultrasonic sensors have a low tolerance for acoustic noise.
One technological approach to solving these problems is the use of guided wave pulses. These pulses are transmitted down a dual probe transmission line into the stored material, and are reflected from probe impedance changes which correlate with the fluid level. Process electronics then convert the time-of-flight signals into a meaningful fluid level reading. Conventional guided wave pulse techniques are very expensive due to the nature of equipment needed to produce high-quality, short pulses and to measure the time-of-flight for such short time events. Further, such probes are not a simple construction and are expensive to produce compared to simple capacitance level probes.
Recent developments by the National Laboratory System now make it possible to generate fast, low power pulses, and time their return with very inexpensive circuits. See, for example, U.S. Pat. Nos. 5,345,471 and 5,361,070. However, this new technology alone will not permit proliferation of level measurement technology into process and storage measurement applications. The pulses generated by this new technology are broadband, and also are not square wave pulses. In addition, the generated pulses have a very low power level. Such pulses are at a frequency of 100 Mhz or higher, and have an average power level of about 1 nano Watt or lower. These factors present new problems that must be overcome to transmit the pulses down a probe and back and to process and interpret the returned pulses.
The present invention relates to a sensor apparatus for transmitting these low power, high frequency pulses down a probe and effecting their return. Currently, no industrially suitable sensor exists which can economically function as a transmission line and withstand typical industrial process and storage environments, while maintaining vessel integrity.
The present invention relates to a single conductor surface wave transmission line (Goubau line) adapted as a sensor for industrial process variable measurement. The present invention incorporates not only the transmission line function, but also a reference pulse means, a sensing function, a process connection mounting function, a sensor fixing means, and a process sealing means all in a single construction which is compatible with standard industrial mounting requirements such as flanges or threaded connections. The apparatus of the present invention also meets the heavy duty demands of an industrial environment and is suitable for installation in areas of high temperature, high humidity, high pressure, high chemical aggressiveness, high static electricity, high pull-down forces in granular materials, and high electromagnetic influence. The sensor apparatus is connected electrically to a process measurement system electronics which provides its power and signal processing. The sensor apparatus is specifically designed to handle high speed, low power, high frequency broadband pulses which are delivered by the system electronics.
The sensor apparatus of the present invention is particularly adapted for the measurement of material levels in process vessels and storage vessels, but is not thereto limited. It is understood that the sensor apparatus may be used for measurement of other process variables such as flow, composition, dielectric constant, moisture content, etc. In the specification and claims, the term "vessel" refers to pipes, chutes, bins, tanks, reservoirs, or any other storage vessels. Such storage vessels may also include fuel tanks, and a host of automotive or vehicular fluid storage systems or reservoirs for engine oil, hydraulic fluids, brake fluids, wiper fluids, coolant, power steering fluid, transmission fluid, and fuel.
The present invention propagates electromagnetic energy down an inexpensive, single conductor transmission line as an alternative to conventional coax (or otherwise dual) cable transmission lines. The Goubau line lends itself to applications for a level measurement sensor where an economical rod or cable probe (i.e., a one conductor instead of a twin or dual conductor approach) is desired. The single conductor approach enables not only taking advantage of new pulse generation and detection technologies, but also constructing probes in a manner similar to economical capacitance level probes.
As discussed above, the simplest implementations of a single transmission line in a process measurement probe will not withstand the previously discussed rigors of an industrial environment. Further, standard capacitance level probes do not accommodate the transmission of high speed pulses due to the electrical impedance discontinuities which exist in their assembly.
The present invention solves problems associated with implementing the new, inexpensive pulse technology by providing an improved mounting, fixing, securing, and sealing sensor apparatus including the combination of a probe element and transmission line. The present invention accomplishes these features while maintaining the electrical operation of a Goubau line including pulse launch, smooth impedance transition from cabling, reference pulse control, transmission through the mounting including both transmitted pulse control and reflected pulse control, and facilitation of desired mode propagation.
According to one aspect of the invention, a sensor apparatus is provided for transmitting electrical pulses from a signal line into a vessel to measure a process parameter. The sensor apparatus includes a housing having upper and lower mounting sections. The lower mounting section is formed to include a tapered surface adjacent the bottom end of the lower mounting section. The apparatus also includes a conductive probe element including a head having a tapered surface and an elongated conductive portion extending away from the head. The tapered surface of the head is configured to engage the tapered surface of the lower mounting section to prevent movement of the probe element in a direction toward the mounting section. A dielectric insert is located above the head of the conductive probe element and a conical transitioning pin has a lower flange configured abut the dielectric insert and a threaded member for coupling the pin to the probe element to secure the probe element to the mounting section.
In the illustrated embodiment, a spring is located between the dielectric insert and a flange of the mounting section to provide an upwardly directed biasing force to the dielectric insert, the pin and the probe element. The apparatus further includes an electrical connector coupled to the conical transitioning pin. The connector is configured to couple the signal line to the probe element.
According to another aspect of the invention, a sensor apparatus is provided for transmitting electrical pulses from a signal line into a vessel to measure a process parameter. The sensor apparatus includes a mounting section configured to be coupled to the vessel and a conductive probe element having a predetermined diameter. The probe element is formed to include a section have a reduced diameter adjacent the top end of the probe element. The probe element includes a tapered section providing transition from the probe element to the reduced diameter section.
In the illustrated embodiment a dielectric insert is located within the mounting section. The dielectric insert includes an inwardly tapered section to prevent movement of the probe element in a direction toward the mounting section and an outwardly tapered section.
Also in the illustrated embodiment, a conductive pin is coupled to the top end of the probe element. The conductive pin has a larger diameter than the diameter of the reduced diameter section of the probe element to engage the outwardly tapered section of the dielectric insert to prevent movement of the probe element in a direction away from the mounting section. An electrical connector is coupled to the pin and is configured to couple the signal line to the probe element through the conductive pin.
According to a further aspect of the invention, a sensor apparatus is provided for transmitting electrical pulses from a signal line into and out of a vessel to measure a process variable. The sensor apparatus includes a mounting section configured to be coupled to the vessel. The mounting section is formed to include a central aperture defined by a first tapered surface. A dielectric insert has a second tapered surface configured to engage the first tapered surface of the mounting section to prevent movement of the dialectic insert in a direction away from the mounting section. The dialectic insert also has a third tapered surface.
In the illustrated embodiment, a conductive transitioning pin has a fourth tapered surface configured to engage the third tapered surface of the dielectric insert.
Also in the illustrated embodiment, a metallic insert is located above the transitioning pin and the dielectric insert. The metallic insert is coupled to the mounting section to secure the dielectric insert within the mounting section. A conductive, flexible or rigid probe element is coupled to the transitioning pin and extends downwardly through the dielectric insert and into the vessel. An electrical connector is coupled to the transitioning pin and is configured to couple the signal line to the probe element through the transitioning pin.
In each of the illustrated embodiments at least one of the transitioning pins and the connector includes an aperture and the other of the transitioning pin and the connector include a pin which slidably engages the aperture to permit movement of the probe element relative to the mounting section while maintaining the electrical connection.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.